How do you make an SSS texture (Using Principled BSDF) only affect part of a mesh? (Image/gif inside)
The best way I can think of is to have Image Texture for Subsurface value which will behave as a SSS intensity control. Where black color = 0 and white = 1. 

In my example I've used such setting with two textures, one for SSS color and another one (black and white) for SSS intensity. This way you can control where SSS will be visible (and how intense).